I don't think that J2's product will become obsolete. I've been using Internet email for 15 years. I've been running an Internet company for 5 years (Server.com). We primarily deal with other Internet companies. These companies always want to send us faxes. If Internet companies still want to use fax machines, how long will it take the mainstream business world to ditch them?
Also, with email attachments, you have to deal with interoperability issues. Someone sent me an MS Word 2000 document attachment. I'm using MS Word '97. What am I supposed to do? Pay Microsoft a few hundred bucks just to read the attachment? I'll take the fax.